The present technology relates to non-volatile memory systems.
Semiconductor memory devices have become more popular for use in various electronic devices. For example, non-volatile semiconductor memory is used in cellular telephones, digital cameras, personal digital assistants, mobile computing devices, non-mobile computing devices, solid state drives, and other devices.
A charge-storing material such as a conductive floating gate or a non-conductive charge-trapping material can be used in some types of memory cells to store a charge which represents a data state. The memory cell may be a transistor in which the charge-storing material may reside between a channel and a control gate. The charge-storing material may be insulated from both the channel and the control gate. The charge-storing material can be arranged vertically in a three-dimensional (3D) stacked memory structure, or horizontally in a two-dimensional (2D) memory structure. One example of a 3D memory structure is the Bit Cost Scalable (BiCS) architecture which comprises a stack of alternating conductive and dielectric layers.
Some types of memory cells may be programmed by altering the charge in the charge-storing material, thereby altering the threshold voltage (Vth) of the memory cell transistor. In one technique, a program voltage is applied to the control gate with a low voltage in the channel to add charge to the charge-storing material. After applying the program voltage, the memory cell's threshold voltage is tested by applying a verify voltage to the control gate, and testing whether the memory cell conducts a significant current. Additional program voltages may be applied to control gate, followed by verify voltages, until the memory cell's threshold current is within a target range.
Other types of memory cells can be programmed to different resistance levels. For example, resistive random access memory (ReRAM) can be reversibly switched between a high resistance state and a low resistance data state. In some memory cells, a ReRAM cell can be reversibly switched between more than two different resistance states. Each resistance state can correspond to a range of resistances.
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